Electron beam dose control for scanning electron microscopy and critical dimension measurement instruments

ABSTRACT

A system and method for controlling electron exposure on image specimens by adjusting a raster scan area in-between scan frame cycles. A small, zoomed-in, scan area and the surrounding area are flooded with positive charge for a number of frame cycles between scan frames to reduce the voltage differential between the scan area and surrounding area, thereby reducing the positive charge buildup which tends to obscure small features in scanned images. The peak current into a pixel element on the specimen is reduced by scanning the beam with a line period that is very short compared to regular video. Frames of image data may further be acquired non-sequentially, in arbitrarily programmable patterns. Alternatively, an inert gas can be injected into the scanning electron microscope at the point where the electron beam impinges the specimen to neutralize a charge build-up on the specimen by the ionization of the inert gas by the electron beam.

FIELD OF THE INVENTION

The present invention relates generally to enhanced feature measurementin scanning electron microscopy, and more specifically to a system andmethods for controlling electron exposure on image specimens in scanningelectron metrology, particularly in the inspection of features ofmicro-circuits. It may also apply to critical dimension measurement insimilar instruments.

BACKGROUND OF THE INVENTION

In scanning electron microscopy, a beam of electrons is scanned over aspecimen, and the resulting electrons that are returned from thespecimen surface are used to create an image of the specimen surface. Insome systems, the beam is arbitrarily controllable to make multiple scanpasses over specific areas or portions of areas at different samplefrequencies to magnify the image of the surface.

On a specimen made of a substantially insulative material (e.g., asemiconductor material), performing multiple scans over the same smallarea may cause the specimen to accumulate an excess positive charge inthat small area relative to the rest of the scanned area. That excesscharge causes an image of that small area to appear dark, thus obscuringimage features in that small area.

SUMMARY OF THE PRESENT INVENTION

One embodiment of the present invention is a system and method forimaging a specimen that acquires a charge when scanned with a scanningelectron microscope comprising an electron source and apparatus forforming, accelerating, focusing, and scanning an electron beam across aportion of said specimen. That imaging being performed by rasterscanning a selected small area of the specimen for a single frame cycle,and then raster scanning a substantially larger area of the specimenthat includes the small area to brighten the image of the small area ofsaid specimen by flooding the substantially larger area with electrons.

A second embodiment of the present invention is a system and method of ascanning electron microscope to image a specimen that acquires a chargewhen scanned with a scanning electron microscope by injecting an inertgas at the point where the electron beam impinges on the surface of thespecimen. That inert gas being ionized by the electron beam and thusneutralizing the charge as it builds up on the specimen.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of thepresent invention and, together with the description, serve to explainthe principles of the present invention.

FIG. 1 is a simplified block diagram of the overall system of thepresent invention.

FIG. 2 a more detailed schematic of the system of the present inventionshown in FIG. 1.

FIG. 3 is an expanded block diagram of the scanning control subsystem ofFIG. 1.

FIGS. 4a-4b show one possible set of scan system voltage control signalwaveforms of FIG. 3.

FIG. 4c illustrates the scan pattern on the substrate when the signalwaveforms of FIGS. 4a-4b are used.

FIGS. 4d-4l illustrate similar scan system voltage control signalwaveforms and scan patterns that may be used with the present invention.

FIG. 5 shows a more detailed view of an image scan area and illustratestypical scan patterns.

FIG. 6 shows a sequence of raster scans to illustrate the chargeflooding technique of one embodiment of the present invention.

FIG. 7 illustrates an alternative detection and imaging subsystem formultiple scans at incremental offset one from the other.

FIG. 8 shows a portion of an image on a specimen to illustrate thecapability of multiple feature measurement of the present inventionwithout having to reposition the specimen.

FIG. 9 illustrates a second embodiment of the present invention thatminimizes the charge build-up in a specimen when being scanned.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 shows a block diagram of system 10 including an electronmicroscope subsystem 11 of the present invention. The electronmicroscope subsystem 11 includes an electron beam source 12, a focusingcolumn and lens assembly 14, and a scan controller 16 to scan anelectron beam across selected regions of specimen 20. Also included inelectron microscope system 11 is an electron detector 24 to detectsecondary and backscattered electrons from specimen 20. In system 10 ofthe present invention, electron detector 24 is selected to have abandwidth that is at least adequate to detect the secondary andbackscattered electrons that form electron signal 28. For example,electron detector 24 may be a micro-channel plate, micro-sphere plate,semiconductor diode, or a scintillator/ photomultiplier (PMT) assembly,each well known in the art. Then the electrons of signal 28 received bydetector 24 are processed and stored for display by image processor anddisplay subsystem 26.

In operation, electron beam 18 is scanned over specimen 20 and secondaryand backscattered electron signal 28 is detected by electron detector24. Further, electron beam 18 is focused on the surface of specimen 20with the average current into specimen 20 determined by scan controller16 that controls the raster scanning of beam 18. In the presentinvention electron beam 18, as discussed below, can be scanned for asingle frame cycle, and then blanked for a period of one or more framecycles.

Typically, specimen 20 may be comprised of a variety of materials withthe present invention particularly applicable to materials containing asubstantial amount of insulative material (e.g., semiconductormaterial). Small area 22 of specimen 20 is shown to illustrate aparticular area of interest to be scanned to determine features of thespecimen in the image of small area 22 developed by image processor anddisplay subsystem 26. For example, small area 22, may, in a degeneratecase, be a single line or a single pixel element on specimen 20. In thepresent invention the peak current onto small area 22 is reduced byscanning electron beam 18 faster than the television rate commonly usedin conventional SEM instruments. In system 10 of the present invention,electron beam 18 is typically scanned with a line period of 16 μsec, orfour times the rate normally used for TV raster scanning having a lineperiod of 64 μsec.

FIG. 2 shows a partial cross-sectional view of electron microscopesubsystem 11 of the present invention to reveal more detail. As shownhere, subsystem 11 is shown with electron beam source 12 at the topwhich produces electron beam 34. One implementation that could be usedincludes an electron gun 36 that consists of a thermal field emitter(TFE) with the electrons accelerated by a surface field generated bypower supply 32. Alternative electron gun embodiments could be employed.The electrons emitted by electron gun 36 are then, within beam source12, directed through electrodes 38 and gun lens 39 (each also controlledby power supply 32) to form electron beam 34 that enters focusing columnand lens assembly 14 to be directed to specimen 20. It should also benoted that electrodes 38 typically include both suppressor and extractorelectrodes.

In focusing column and lens assembly 14, electron beam 34 passes throughan aperture 41, reducing the beam current from approximately 300 pA to arange of 5 to 100 pA forming what is labelled electron beam 34' in FIG.2. A larger electron beam current (e.g., 100 pA) is particularly usefulfor pattern recognition. That larger beam current also reduces theintegration time to achieve a given signal-to-noise ratio for the imageor linescan which is well known in the art. Stated a little differently,there is a better signal-to-noise ratio for higher beam currents,however there is an improved image quality for lower beam currents.

Electron beam 34' then passes through objective lens 42, includingmagnetic coils 43 and pole pieces 44, that generate a strong magneticfield. That magnetic field is used to focus beam 34' to form electronbeam 18 with a spot size of approximately 5 nm when directed at specimen20. Additionally, the location of electron beam 18 is controlled withscan plates 45, located within the magnetic field created by coils 43and pole pieces 44, with scan plates 45 powered by raster generator 48to direct beam 18 in both the x and y directions across specimen 20 bysignals on lines 46 and 47, respectively. To tie FIGS. 1 and 2 togetherin this area, scan plates 45 and raster generator 48 correspond to scancontrol 16 in FIG. 1.

Referring next to FIG. 3, there is shown a block diagram of onepotential embodiment of raster generator 48. Included in this sampleembodiment of raster generator 48 is a clock 60 to produce a timingsignal that is applied to ramp generator 62 and counter 64. Rampgenerator 62 in turn produces a ramp signal x', and counter 64 producesa digital signal which represents a preset count. The preset count fromcounter 64 being representative of the timing signal from clock 60. Inturn, the preset count from counter 64 is applied to look-up table 65wherein look-up table 65 has been programmed to select individual y-axislines on the surface of specimen 20 to be scanned that corresponds tothe count from counter 64. It should be noted here that the y-axis linesto be scanned may be sequential; non-sequential; selected lines with oneor more intermediate lines skipped; selected lines scanned repeatedly;or any combination or order desired for various regions on the surfaceof specimen 20. The output digital value of look-up table 65 is thenapplied to digital-to-analog converter (DAC) 66 to produce a steppedsignal, y', that corresponds to the y-axis position on specimen 20 to bescanned. Next, signals x' and y' are directed to the rotation andscaling controllers 68 (e.g., utilizing a multiplying D/A converter witha technique that is well known in the art) that produces signals x and ythat are applied to scan plates 45 (see FIG. 2) via lines 46 and 47,respectively, to control the actual x and y positions electron beam 18scans on specimen 20.

Referring next to FIGS. 4a and 4b, representative waveforms of signals x(46) and y (47), respectively, from raster generator 48 are shown. InFIG. 4a, ramp segment 72 in the x signal (46) directs beam 18, via scanplates 45, to scan a spot along a single line in the x-axis direction onspecimen 20. Since each segment of the signal in FIG. 4a is the samemagnitude in voltage, alternatively the same duration in time, thelength of each corresponding scan in the x direction is of the samelength. Concurrently, in FIG. 4b each step segment 76 of the y signal(47) provides a y-address of a different signal value in the y-axisdirection that is traced in the x direction of specimen 20 by the xsignal. To illustrate what the x and y signals of FIGS. 4a and 4b areactually causing to happen relative to specimen 20, FIG. 4c is providedto show the paths scanned based on those signals, i.e., each line startsat x_(o) and proceeds to x_(a) at each of the corresponding ycoordinates starting with y_(a) and progressing through y_(e).

However, scan signals x and y may be manipulated to vary the scanpattern in various ways. For example, in FIGS. 4d and 4e, each of linesy_(a), y_(b) and y_(c) on specimen 20 are each scanned twice along the xaxis from x_(o) to x_(a), and then back from x_(a) to x_(o) beforeprogressing to the next y line.

Another potential scan pattern is represented by FIGS. 4f and 4g whereline y_(a) is scanned three times in the x direction between x_(o) andx_(a), always in the same x direction; then line Y_(b) is scanned oncebetween x_(o) and x_(a) ; next line Y_(c) is scanned twice in the xdirection between x_(o) and x_(a), always in the same x direction; andfinally each of lines Y_(d) and Y_(e) in that sequence, are scanned oncebetween x_(o) and x_(a).

Yet another scan example is illustrated FIGS. 4h, 4i and 4j. Here two ylines, Y_(a) and Y_(b), are scanned in sequence with each y line scannedonce between x coordinates x_(a) and x_(b), and then twice between the xcoordinates x_(o) and x_(c).

An additional example is illustrated in FIGS. 4k and 41, assuming thatthe x axis scan is as illustrated in FIG. 4a. What is shown here is thenon-sequential scanning of substrate 20 along a group of y lines, y_(a)through y_(g), in the order of: y_(c), y_(f), y_(b), y_(d), y_(g), y_(a)and y_(e).

It should be kept in mind that the various scan patterns illustratedhere simply illustrate the variations in scan patterns that can be usedand they are not intended to be anything other than examples of thevariations of scan patterns that may be used.

Returning to FIG. 2, as beam 34' passes through the magnetic field ofobjective lens 42 and plates 45 it is focused into beam 18 and directedonto specimen 20. Given tolerances in today's applications, the spacingbetween column 14 (bottom of lens 42) and specimen 20 will typically beon the order of 2 mm, however that spacing is not critical to theoperation of the present invention, it merely must be a known value. Inaddition, specimen 20 is biased to a selected potential by a secondpower supply 52 (e.g., up to 5VDC) to create an extremely largedecelerating field for the primary electrons of beam 18 as they approachspecimen 20. The result is that the "landing energy" of those electronsas they reach specimen 20 is therefore much lower than the energy withwhich they are provided by electron gun 36 and with which they travelthrough column and lens assembly 14. The electron beam of theillustrated implementation starts out from electron gun 36 with anenergy level of typically 5000 eV, and travels through column and lensassembly 14 with that energy level essentially unchanged. As electronbeam 18 exits lens 42, the decelerating field radiating from specimen20, created by the bias of second power supply 52, substantiallydecelerates the electrons within beam 18 to the desired landing energy.

The effect of reducing the landing energy of the electrons bycontrolling the decelerating field allows for excellent opticalperformance by reducing the chromatic aberration coefficient ofobjective lens 42, and provides some immunity from stray magnetic fieldsin the environment (e.g., stray fields of 50 or 60 cycles from powerlines). Thus, the beam landing energy can be adjusted by adjusting thebias applied to specimen 20 from second power supply 52.

Continuing the discussion of the operation of the system illustrated inFIG. 2, secondary and backscatter electrons 28 are released as a resultof the interaction of electron beam 18 with specimen 20 and are directedback toward lens 42. As electrons 28 are released, they spiral throughlens 42 as a result of the magnetic field, and then travel towarddetector 55 as they leave the field within lens 42. The electron signalreceived by detector 55 is then collected by collector plate 56 whichin-turn generates a signal that is amplified by transimpedance amplifier58 before being applied to image generator 59. Other input signals toimage generator 59 are signals x and y from raster generator 48 on lines46 and 47, respectively, to form a video signal representing an image ofspecimen 20, or selected portions thereof. Again correlating therelationship between FIGS. 1 and 2, electron detector 24 includesdetector 55 and collector plate 56, while image subsystem 26 includesamplifier 58 and image generator 59. Additionally, electron beam source12, focusing column and lens assembly 14, and specimen 20 are allcontained within a vacuum chamber 23.

Note also that when a high electron beam current 18 is used, theintegration time for detector 55 to achieve a given signal-to-noiseratio for an image or linescan is reduced. This shorter acquisition timeallows faster pattern recognition in automated systems, and reducessensitivity to low frequency vibration and electronic andelectromagnetic noise in the system.

In a system as described herein it is useful to look at the ratio of thedetected electron beam current 28 from specimen 20 to the incomingelectron beam current 18 to specimen 20, with that ratio referred to asthe "emission coefficient". There are several variables that affect thevalue of the emission coefficient, some of which are the specimenmaterial, the topography of the sample area, the bias voltage on thespecimen and the landing energy of the primary electron beam. In caseswhere the emission coefficient is greater than one (e.g., for siliconspecimens) -- that is, more electrons are being generated at the scannedarea than are arriving at it -- the specimen tends to build up apositive charge in the scanned area. For other materials the emissioncoefficient will have differing values, greater than, less than, orequal to, one when a positive charge builds on a specimen of thatmaterial. The field which decelerates the primary beam (i.e., resultingfrom the bias of second power supply 52) further tends to accelerate theelectrons of beam 28 as they leave the specimen surface, whichaccentuates the depletion of electrons from specimen 20.

As mentioned previously, the electron microscope of the presentinvention is able to select small areas, including a single line, forraster scanning. Incoming electron beam 18 is further controllable sothat any particular line or area on specimen 20 may be scanned severaltimes. This creates a problem, however, in scanning situations where theemission coefficient is greater than one (e.g., for silicon specimens),or for whatever value for other materials that might constitute specimen20. Attempting to zoom in on an image and measure very small areasresults in the accumulation of a large positive charge in that area, andelectrons are prevented from escaping from specimen 20 by the resultingelectrostatic field. In the present invention, this problem is solved byflooding the surrounding area with electrons during a number of framecycles as discussed below.

As shown in FIG. 5, a small area 22 (see FIG. 1) may be scanned line byline. In the present invention that scan could begin with electron beam18 at a top left pixel 102, proceed to the right across that y line inthe increasing x direction to pixel 104, then proceed downward to apixel 105 in another y line with the same x coordinate, from thereproceed to the left in the negative x direction across that new y lineto pixel 106, and continue scanning in that back and forth fashion invarious y lines across small area 22. Then, when that scan reaches pixel112, the beam is "blanked" (i.e., temporarily turned off) while electronbeam 18 is returned to pixel 102. Alternatively, the scan may becontrolled in one of the alternative patterns discussed in relation toFIGS. 4d-4l-- what ever is appropriate for specimen 20.

On a specimen made up of a substantially insulative material (e.g., asemiconductor die), each scan may result in the release of secondaryelectrons, increasing the positive charge of the area of interest witheach scan. As a result of repeated scans, small area 22 acquires ahigher positive charge than the surrounding area of specimen 20. Such apositive charge will be displayed as a darkened area by image processorand display subsystem 26 in the resulting image. Depending on the levelof positive charge on small area 22 relative to the surrounding area,features of small area 22 may be difficult to discern in that image.

As shown in FIG. 6, the technique of one embodiment of the presentinvention alleviates that darkened image problem by performing asequence of scans which includes flooding an image area 120 (includessmall area 22 and the area surrounding small area 22) during an integernumber of raster scans. For example, in a first scan frame cycle, n₁,each line of only small area 22 is scanned. In each of a selected numberof subsequent frames n₁ +1 , n₁ +2, . . . , n₁ +m, each line in all ofimage area 120 is scanned in each frame sequentially, each time scanningthe significantly increased image area of image area 120 as compared tosmall area 22, thus essentially flooding the image area 120. The nextsmall area 124 on specimen 20 (may be the same, overlapping, adjoining,or separated from, small area 22) and the surrounding larger image areais similarly scanned in frame n₂. This process is thus repeated untilall of the small areas 22, 124, . . . , of interest are scanned. It mustalso be kept in mind that each subsequent small area to be imaged may bethe same as the previously scanned small area, or offset from thatpreviously scanned small area. Also during scanning of subsequent smallareas (e.g., small area 124), the image area (e.g., 120') to be flooded,may include a substantial portion of the image area (e.g., 120) of thepreviously imaged small area (e.g., 22) since at least the image areas,if not the small areas as well, may overlap each other.

Flooding the scanned small area and surrounding image area with positivecharge effectively reduces the voltage differential between the smallarea (e.g., 22) to be imaged and the surrounding image area (e.g., 120less 22), thus allowing electrons to continue to escape from the imagedsmall area. The overall charge that builds up on specimen 20 whileimaging each small area can be adjusted by changing the ratio betweenthe number of frames in which only the small image area is scanned(zoomed-in-frame) versus the number of frames during which the largerimage area is scanned (zoomed-out-frame).

FIG. 7 provides a sample implementation of an electron detector andimage processor subsystem 128 that performs the combined function ofelectron detector 24 and image processor and display subsystem 26 ofFIG. 1. Specifically, subsystem 128 includes a detector 130 that detectsthe reflected and backscattered electrons from specimen 20 with theoutput signal from detector 130 applied to amplifier 132. Amplifier 132subsequently supplies an amplified signal to digitizer 134 where thesignal is digitized for application to image processor 136. In the lowerpath of subsystem 128 there is an oscillator 138 that applies a signalto frequency divider 140 to generate signals to control both digitizer134 and image processor 136 with the operation of subsystem 128discussed more completely below. Additionally, comparing the componentsillustrated in FIG. 7 to those shown in FIG. 2: detector 130 relates todetector 55 and collector plate 56; amplifier 132 relates to amplifier58; and the remainder of the circuit in FIG. 7 relates to imagegenerator 59.

In the embodiment illustrated in FIG. 7, each y line scan signal in anarea of interest on specimen 20 is strobed at four times theconventional video rate (i.e., 160 MHz, the frequency of oscillator 138,corresponds to a four times interleaving using a standard video rate).In the lower path of subsystem 128 a 160 MHz signal is generated byoscillator 138 and applied to frequency divider 140 that performs twofunctions.

One function of frequency divider 140 is to divide the 160 MHz signal byfour to present a 40 MHz signal to image processor 136. The secondfunction of frequency divider 140 is to phase split a 160 MHz signalfrom oscillator 138 into four 160 MHz strobing signals, each with adifferent phase relative to each other (i.e., φ₁ =0°, φ₂ = 90°, φ₃ =180° and φ₄ = 270°). Each of those four different phase strobingsignals, φ₁, φ₂, φ₃ and φ₄, are applied to a different terminal ofdigitizer 134 to cause digitizer 134 to divide each y line scan signalfrom amplifier 132 into four different y line scan signals.

Given this embodiment, 512 subpixel samples are obtained during eachphase shifted strobe, and four consecutive strobes of the same y linescan signal are each strobed with an incremental offset of phase (i.e.,a quarter pixel width). These phase shifted y line scan signals areinterlaced, resulting in a total of 512 times 4, or 2048 samples perline (e.g., when there are a total of 2048 sample pixels for a y linescan signal, the first strobe with a 0° phase shift strobes pixels 0, 4,8, etc.; on the next strobe with a 90° phase shift pixels 1, 5, 9 etc.are strobed; on the next scan with a 180°phase shift pixels 2, 6, 10etc. are strobed; and on the fourth scan with a 270° phase shift pixels2, 7, 11 etc. are strobed).

The four phase implementation discussed with respect to FIG. 7represents an economical way of extracting the data from the y line scansignals using less expensive 10 MHz equipment rather than 40 MHzequipment that would be needed without the strobing routine.

FIG. 8 illustrates one of the aspects of the system of the presentinvention in which arbitrary programming of the direction of a linescan, and non-sequential line scans, can be used to obtain severalcritical dimension measurements on a substrate without having toreposition the substrate between each measurement.

Before illustrating that, it would be helpful to introduce the conceptof "charge induced asymmetry". Basically when a feature is scanned, suchas a line on a wafer, the video signal from the leading edge of thatscan provides a different image than the trailing edge of that scan as aresult of the scanning process depositing a charge on the wafer duringthe scanning process thus affecting the resultant video image. Thatdifference in image is referred to as "charge induced asymmetry".

During the development of the present invention it was observed thatline scan direction reversal during scanning reduces charge inducedasymmetry in the line scan profiles. Therefore, multiple arrays of linescan data may be acquired, wherein the position, length, and orientationof each line scan over the specimen is arbitrarily programmable.

Specifically FIG. 8 shows a portion of a conductive trace 150dog-legging around a conductive pad 152 on an insulative material. Scanlines 154 and 156 have been added to illustrate two potential locationswhere multiple, independent measurements may be made sequentiallywithout having to reposition the substrate. Stated in another way, thescanning control system of the present invention can be programmed todeflect the electron beam to separated regions of the portion of thespecimen surface beneath the beam deflection window of the electron beamcolumn without moving the specimen. This ability to average over verysmall areas, and over different orientations, allows for accurate rapidmetrology directly off the segment, without repositioning the sample.

Another embodiment of the present invention to neutralize the chargebuild-up on a substrate during scanning, and the resultant darkenedregion is shown in FIG. 9. Enclosed within vacuum chamber 200 areelectron beam source 12, focusing column and lens assembly 14 andspecimen 20 as in FIG. 2. Additionally, a capillary tube (or capillaryarray) 202 (such as made by Galileo of Sturbridge, Ma.), made of anelectrically conductive material, is inserted into chamber 200 betweenlens assembly 14 and specimen 20 with an orifice of tube 202 directed atthe point where the electron beam impinges on specimen 20. External tovacuum chamber 200 is supply tank 206 to contain an inert gas (e.g.,argon) for delivery to capillary tube 202 via the serial connection ofleak value 204 (e.g., Varian Model 951-5106) and a gas supply tube 208.The purpose of leak valve 204 is to control the rate at which the inertgas is injected into chamber 200 to maintain the vacuum at the desiredlevel (e.g., 10-⁴ Torr). Thus, by injecting the inert gas into chamber200 at the point at which the electron beam scans specimen 20 the gasionizes and in so doing neutralizes the charge build-up on specimen 20.

Although the invention has been described in relation to variousimplementations, together with modifications, variations, and extensionsthereof, other implementations, modifications, variations and extensionsare within the scope of the invention. Other embodiments may be apparentto those skilled in the art from consideration of the specification andinvention disclosed herein. The invention is therefore not limited bythe description contained herein or by the drawings, only by the scopeof the claims.

What is claimed is:
 1. A method for imaging a specimen with a scanningelectron microscope comprising an electron source and apparatus forforming, accelerating, focusing, and scanning an electron beam across aportion of said specimen, wherein said specimen is subject to a chargebuild-up thereon when so scanned, said method comprising the steps of:a.raster scanning a first substantially large area of said specimen thatincludes a first small area of said specimen with said electron beam;and b. raster scanning said first small area of said specimenindividually with said electron beam following step a. to brighten theimage of said first small area of said specimen as a result of theflooding of said first substantially large area with electrons.
 2. Amethod as in claim 1 wherein step a. is repeated a plurality of times tobrighten the image of said first small area of step b. relative to acumulative image of said first substantially large area of step a.
 3. Amethod as in claim 1 wherein said first substantially large area of stepa. surrounds said first small area thus the area surrounding said firstsmall area is flooded with electrons.
 4. A method as in claim 1 furtherincluding the steps of:c. raster scanning a second substantially largearea of said specimen that includes a second small area of said specimenwith said electron beam; and d. raster scanning said second small areaof said specimen with said electron beam following step c. to brightenthe image of said second small area of said specimen as a result of theflooding of said second substantially large area with electrons.
 5. Amethod as in claim 4 wherein steps a. and c. are each repeated aplurality of times to brighten the image of each of said first andsecond small areas of steps b. and d. relative to cumulative images ofeach of said first and second substantially large areas of steps a. andc.
 6. A method as in claim 4 wherein said first substantially large areaof step a. surrounds said first small area, and said secondsubstantially large area of step c. surrounds said second small areathus flooding the areas surrounding each of said first and second smallareas with electrons.
 7. A method as in claim 4 wherein said first andsecond small areas at least partially overlap each other.
 8. A method asin claim 4 wherein said first and second substantially larger areas atleast partially overlap each other.
 9. A method as in claim 4 wherein aline period of each raster scan of each of said first and second smallareas of steps b. and d. is substantially shorter than typical videoscan rates of scanning electron microscopes to reduce the peak scanningcurrent.
 10. A method as in claim 1 wherein a line period of each rasterscan of said first small area of step b. is substantially shorter thantypical video scan rates of scanning electron microscopes to reduce thepeak scanning current.
 11. A method as in claim 1 wherein step b. isperformed by scanning said electron beam across said first small areastarting at a selected first x axis coordinate and ending at a second xaxis coordinate for each of a selected number of y axis coordinates withsaid electron beam otherwise blanked.
 12. A method as in claim 1 whereinstep b. is performed by scanning said electron beam across said firstsmall area starting at a selected first x axis coordinate, proceeding toa second x axis coordinate and returning to said selected first x axiscoordinate for each of a selected number of y axis coordinates with saidelectron beam otherwise blanked.
 13. A method as in claim 1 furtherincluding the step of:e. strobing each y axis scan signal of steps a.and b. at a selected multiple of the video scan rate to convert eachsaid y axis scan signal into selected multiple y axis scan signals withan equal phase shift between each of said selected multiple y axis scansignals being determined by the multiple selected.
 14. A method as inclaim 13 wherein said selected multiple in step e. is four resulting insaid selected multiple y axis scan signals being four in number eachwith a phase shift of 90° between each subsequent one of said fourselected multiple y axis scan signals.
 15. A method for imaging aspecimen with a scanning electron microscope comprising an electronsource and apparatus for forming, accelerating, focusing, and scanningan electron beam across a portion of said specimen, wherein saidspecimen is subject to a charge build-up thereon when so scanned, saidmethod comprising the steps of:a. raster scanning a selected small areaof said specimen for a single frame cycle; and b. injecting an inert gasat a point above said specimen where said electron beam impinges on saidspecimen to neutralize a charge build-up on said specimen by theionization of said inert gas by said electron beam.
 16. A method as inclaim 15 wherein said inert gas is argon.
 17. A scanning electronmicroscope to image a specimen having a secondary emission energy levelthat is greater than an energy level of a scanning beam of said electronmicroscope, said scanning electron microscope comprising:a vacuumchamber; an electron source contained within said vacuum chamber; afocusing column and lens assembly contained within said vacuum chamberto direct electrons from said electron source to said specimen containedwithin said vacuum chamber; a secondary and backscatter electrondetector contained within said vacuum chamber to detect electrons fromsaid specimen; and a capillary tube extending into said vacuum chamberwith an orifice positioned above said specimen at a point whereelectrons from said focusing column and lens assembly are deliveredthereto, said capillary tube disposed to be connected to an inert gassupply tank external to said vacuum chamber to inject an inert gas fromsaid orifice.
 18. A scanning electron microscope as in claim 17 whereinsaid inert gas is argon.